Intrinsic Safety

Hazardous area Non Hazardous area

Training Development and Delivery

1. Principles of Intrinsic Safety

2. Application of Intrinsic Safety

3. Installation

4. Inspection


THE IGNITION TRIANGLE

Fuel

Note : Air has approximately 21% oxygen

IGNITION SOURCEOXygen

EXPLOSIONEXPLOSION


Plant is classified according to :

* the probability that the Hazardous Atmosphere will be present

AREA CLASSIFICATION

* the nature of the Hazardous Atmosphere

GAS GROUPING

( in the UK these are Zone 0, 1 & 2 )

( IIA , IIB and IIC )


Representative Gas for Sub-Division of Groups IIA , IIB and IIC

Group IIA Propane Energy level for Ignition 160J

Group IIB Ethylene Energy level for Ignition 95J

Group IIC Hydrogen Energy level for Ignition 20J


Area Classification - Recap

Probability of Gas/Air

Mixture being

present

Zone 0,1, 2

X

Probability of source of

ignition

Matched to

Type ofProtection

d , e ,n, I

=

Acceptable Risk

Nothing is 100% safe


A system, comprising apparatus and interconnecting wiring, in which any spark or thermal effect in any part of the system intended for use in hazardous areas is incapable of causing ignition.

Intrinsically Safe System

By Electrical Design


Power supply Conventional Power Circuits

What happens when a fault occurs?


12V Car Battery

conductor resistance 0.5

12V headlamp

I = V/R

I = 12/1

I = 12 A

P = V x I

P = 12 x12

P = 144W

Energy W = P x t

Energy W= 144 x 0.01

Energy W = 1.44 Joules


Short circuit fault


12V Car Battery

conductor resistance 5

12V headlamp

I = V/R

I = 12/10

I = 1. 2 A

P = V x I

P = 12 x1.2

P = 14. 4W

Energy W = P x t

Energy W = 14. 4 x 0.01


Energy W = 144mJ


Short circuit fault


12V supply

Current limiter of 20k

TransducerI = V/R

I = 12/20000

I = 0.00072 A

I = 0.72mA

P = V x I

P = 12 x 0.00072

P = 0.0072W

P = 7.2mW

Energy W = P x t

Energy W = 0.0072 x 0.01


Energy W = 72J

Load

Training Development and DeliveryTraining Development and Delivery

How can the principles of Intrinsic Safety be achieved ?

Careful design of circuits which limit the amount of available electrical energy

Where can the principles of Intrinsic Safety be applied ?

Due to the small levels of available energy , this method of protection is onlysuitable for instrumentation or control circuits

How can the principles of Intrinsic Safety be maintained ?

By regular planned inspection and maintenance


Approx . 140mA

140mA . Applying a safety factor Of 1.5 , gives approx 93mA


For a 28V Zener with a300 ohmn resistorfor use in a IIC area

Apply a 10% safetyfactor for zenervoltage=30.8V

28V/0.093mA= 300 ohmsBarrier is suitable formaintaining theintegrity of the IS circuit in aIIC hazard

12 V

PSU240VRelay12 V 400

Hazardous area

Low power circuit - IS or non - IS ?

I = V/R I = 12/400

I = 0.03A I = 30mA


What if the relay coil inductance created a spark at the contactscapable of causing ignition?

Low power circuit - Insertion of an energy limiting network

12 V

PSU240VRelay12 V 400

Hazardous area

Limitingnetwork


Limit Voltage

i

Limit Current Limit stored electrical energy


Ex iaIntrinsically safewith two faults

Zones 0, 1 and 2

Ex ibIntrinsically safewith one fault

Zones 1 and 2

"Faults" are those in components upon which the safety of the installation depends

Intrinsic Safety is the only protection concept which considers failure of the field wiring


AND THEIR APPLICATIONAND THEIR APPLICATION


800 700 600 500 400 300 200 100

Reverse voltage ( Volts )

0.6 0.7 0.8

Forward voltage ( Volts )

A Forward current

mA Reverse current

Forward bias conducting voltage of 0.6V

Negative

Positive

Forward Bias

Positive

Negative

Reverse Bias

SILICON DIODE CHARACTERISTICS


Hazardous area connection

Non Hazardous area connection

Resistors are usually wire wound and are assumed to fail open circuit

Vz

Fuse

Any load connected in parallel with the Zener Diode will maintain the same potential




Using 2 diodes in this example indicates that the device is safe with one fault

A device like this would have the category ?

Vz

Fuse

i b




Using 3 diodes in this example indicates that the device is safe with 2 faults

A device like this would have the category ?

Vz

Fuse

ia


Fuse rating is chosen to protect the diodes and is not directly related to the barrier short circuit current

CENELEC and other standards require the fuse to be encapsulated within the barrier so that the fuse is not replaceable

Fuse


Fuse

Secondary replaceable fuse versions of barriers are available. These are useful where there is a possibility of faults occurring during commissioning which would otherwise blow the barriers' internal safety fuses.

One secondary replaceable fuse for each barrier channel is provided and is lower in value than the related safety fuse. Fuses are packaged in small mouldings which can be latched in a 'disconnect' position to break the safe and hazardous areas during commissioning, maintenance or fault finding, avoiding the need for additional disconnect terminals.


Fuse

Vz

If the barrier is used the wrong way round, then the fuse will no longer protect the Zener.

Hazardousarea connection



Barriers are usually described in terms of their safety parameters: 28V 93mA 300 is a common barrier safety description

Where Vz = 28 V R = 300

28 V is the maximum OPEN Circuit voltage available at the hazardous area terminals and 93 mA is the maximum short circuit current available.

Fuse

Vz


1

2

Recap Questions 1

Q1. A safety factor of 1.5 is applied to components designed to be used for Intrinsically safe devices . True / False.

Q2. An intrinsically safe system identified as ( ib ) will remain safe with 2 faults . True / False.

Q3. The fuse encapsulated within a zener barrier is designed in relation to the barrier short circuit current . True / False .

Q4. An Intrinsically safe instrument designed for operation in the field would be connected to terminals 1 and 2 of a zener barrier . True / False.


Intrinsically Safe System

AssociatedElectricalApparatus

Intrinsically SafeElectricalApparatus

Field wiring

Intrinsically SafeElectricalApparatus

AssociatedApparatus

Intrinsically SafeElectrical Apparatus

Non-hazardous area Hazardous area

Intrinsically Safe Circuit

Note:

An intrinsically safe circuit consists of all the intrinsically safe apparatus in the loop PLUS the cable


The Entity Concept

In the entity concept each item possesses its own certificate ofconformity that provides sufficient information to enable end users toassemble intrinsically safe systems.


Safe area limitations

InstrumentElectronics

Powersupply

250 VRMS.max.

Uncertified instrument

RequirementsSuitably fused double wound mains transformer

Should not be fed from or contain voltages greater than 250V rms.

P

N

E


Intrinsically safe systems

Certifiedhazardous

areaapparatus

Uncertifiedsafe areaapparatus


Certified interface remote from the uncertified safe area apparatus


Intrinsically safe systems - typical installation

Uncertified safe areaapparatus

Non Hazardous area

Certifiedinterface

Hazardous area

Junction BoxField Apparatus


Intrinsically safe systems

SimpleApparatus



Certifiedinterface


Hazardous area equipment in an intrinsically safe loop is either

Certified energy storing instruments etc.or

Simple apparatus non-energy storing devices

Simple apparatus: Devices in which, according to the manufacturer's specifications, none of the values 1.5V, 100mA, or 25 mW is exceeded, need not be certified or marked


Piezodevice1.2 V

Piezoelectric device limited to 1.2 V outputDoes this still meet the simple apparatus criteria?


Piezodevice1.2 V

Simple apparatus has to be inherently simple.

You cannot make apparatus simple by the addition of a limiting device, which could fail.


Hazardous (Classified) Location Non-Hazardous Location

Monitoring

Instrumentation

110 V

Thermocouple

mVNormalOperation

Bad Day

Monitoring

Instrumentation

110 V

Thermocouple

110 V


Intrinsically safe systems ( REMINDER )

Non-Certifiedhazardous area

Simple apparatus


Non Hazardous area

Certifiedinterface

hazardous area



Monitoring

Instrumentation

110 V

Thermocouple

mV

Intrinsically safe interface device


Hazardous Location Non-Hazardous Location

Simple apparatus•Thermocouples•Photocells•RTDs

Intrinsicallysafe apparatus•Transmitters•Solenoid Valves

Intrinsically safe

interface

Controlroomequipment

AssociatedApparatus

• Zener Barrier• OPTO Isolator


Barriers are required to be connected to the main system electrical earth with a resistance of less than one Ohm

Main electrical system earth point is usually the neutral star point

The important resistance is between the barrier earth rail and the main earth busbar connection point

Minimum CSA of conductor is 4mm2 copper . However ,standards indicate that TWO copper conductors of 1.5mm2 can also be used.


Shunt-diode safety barrier installation

Safearea

EquipmentDistributiontransformer

IS earth (<1 )

Barrier Fuse

For safety reasons the connection to the neutral star point must be of low resistance and secure

ONE 4mm2 conductor or TWO 1.5mm2 parallel conductors


Shunt-diode safety barrier under safe area mains fault

Safearea


IS earth (<1 )

Fault

For safety reasons the connection to the neutral star point must be of low resistance and secure


ApparatusIntrinsically Safe Hazardous Area apparatus falls into one of two categories

• Portable Apparatus

• Field Mounted Apparatus

Equipment of this nature is typically marked EEx ia IIC T4


The Intrinsically Safe Continuityand Insulation Resistance Tester

METROHM

PORTABLE APPARATUS


Apparatus

• Intrinsically Safe Non Hazardous Area Apparatus

Equipment of this nature is typically marked [EEx ia] IIC


Certificate of Conformity

Example

Z barrier CE II 1 GD [EEx ia] IIC CE ll 3 G EExn A ll T4

Applies to APPARATUS certified by an EC approved test Authority to a CENELEC standard recognised by the EC Directives

Will Include the EEC Distinctive Community MarkApproved code will be EEx


Shunt-diode safety barrier under hazardous area mains fault

Safearea


Fault

Raisedpotential

JB

Plant earth


Recap Questions 2

Q1. A device marked EEx ib IIB T4 is suitable for mounting in a Zone 1 hazardous area where Ethylene gas is being processed . True / False.

Q2. The minimum size of earthing lead permitted for the connection from plant earth to the I.S. earth is a 6.00 mm2 copper conductor. True / False .

Q3. It is permitted for the resistance / impedance of the earthing lead to be greater than 1True / False.



(L & C parameters)

Assume an interface of 28V 300 ohms resistanceMax’m source current = 28/300 = 93.33 A (IIC gas area)Apply safety factor of 1.5 Max’m source current = 140 maFrom ignition curves the maximum safe inductance for Group llC is found to be 4.0 mH

For Capacitive circuits apply safety factor of 1.5 to the zener barrier voltage of 28V = 42V For a IIC gas the maximum safe capacitance is 0.08 microFarads


Segrataion of IS and non Is circuits in hazardous areas is essential to avoid the possibility of higher voltages invadingEExi circuits

Cable screens should be earthed at one point only (zener barrier)

Cable marking ---Marking of Is cables is not necessary if either theIs or non-Is cables are armoured, screened or metal sheathed


Is and non Is circuits must not be run in the same multi -core cable

Multi core cables must be capable of withstanding a 750V dc IR test

Installation must comply with system documentation and the manufacturer's recommendations

Location of the interfaces should be permanently marked to show the correct type of replacement barrier in each position

Barriers are normally mounted in the safe area at the nearest convenient point to the hazardous/safe area boundary

Hazardous area mounting is permissible if the appropriate type of protection is provided for the barriers and cabling :

Zone 1 : Flameproof (Ex d) enclosureZone 2 : Type n enclosure


Mechanical protection not necessary for safety purposes

Mechanical protection not necessary for safety purposes

The cable must:

Be capable of withstanding a 500 V insulation test to earth for 1 minute

Conform to the cable parameter requirements

Be clearly identified as carrying intrinsically safe circuits by the use of

Blue cable Blue tape

Be suitable for environmental conditions

The cable must:

Be capable of withstanding a 500 V insulation test to earth for 1 minute

Conform to the cable parameter requirements

Be clearly identified as carrying intrinsically safe circuits by the use of

Blue cable Blue tape

Be suitable for environmental conditions


Multicore Cables are permitted in IS systems but IS and non-IS circuits must not be contained in the same multicore

Multicores

should be run where the risk of mechanical damage is slight should be fixed throughout their length each IS circuit to occupy adjacent cores no voltage should exceed 60V peak

Outer sheaths need not necessarily be blue for IS systems but easy identification of IS cables is required.


X

R

R

NON - HAZARDOUS AREA HAZARDOUS AREA

CPC

CPC

Connected to plant earthvia gland at detector

Screen , insulated and leftto float in detector box

Screen connected to plant earth at detector

Connected to plant earthvia gland at enclosure

I.S. earth terminal

Break in screen when carrying earth leakage current

SPARK

Cable Screen EarthingRequirements


0 0

0 0

0 0

HAZARDOUS AREA

NON-HAZARDOUS AREAIS Earth ( Min csa 4.00mm2 )

BARRIER ENCLOSURE

JOINT BOXScreen insulated and leftto float in enclosure

Screen connected to through terminal . Isolated from plant earth.

Screen connected to barrier busbar

CPC connected via gland body

TRANSDUCER

SCREEN EARTHING ARRANGEMENT


ISCircuits

Separate cable trays.

Partition must be earthed.

IS and non-IS circuits can be in the same tray so long as at least one cable is armoured or has a metal sheath.

non-ISCircuits

ISCircuits

non-ISCircuits


Earthing where fault currents are caused to flow via a dedicated return path (backed up by connection to physical ground) in such a way as to operate a protective device in an appropriately short time.

Bonding where voltage difference between parts of plant, handrails etc., are eliminated by physical cross-connection ( i.e. common potential )


The length of the cable in the hazardous area is the only crucial factor when using simple apparatus and an approved safety barrier

The length of the cable in the hazardous area is the only crucial factor when using simple apparatus and an approved safety barrier


Safety barrier


Certified system

31/2 digitIndicator

MTL 633B

EEx ia IIC T4Ex94C2053

4 - 20 mA

Safety description

Input terminals: non-energy storing apparatus; can be connected without further certification into any IS loop


Least expensive no requirement for lockable fused isolators, protected cable, special glands

Simple apparatus permits the use of normal industrial devices if they are non-energy storing

Fault Tolerant IS is the only technique which remains safe after faults develop in cables and fallible components

Live maintenance IS is the only technique which permits live working without gas clearance certificates

Unarmoured cables System is electrically, not mechanically protected

Safe for personnel extra low voltage and currents


Recap Questions 3

Q1. Cable screens should be insulated and left to float in the enclosure of a transducer in the hazardous area . True / False .

Q2. Intrinsically safe circuit cables should be tested with a potential of 500V to determine if the insulation has been damaged . True / False .

Q3. 6mm is the minimum clearance between similar I.S. circuits . True / False .

Q4. 100mm is the minimum distance for segregation between I.S. and Non I.S circuits . True / False .


Screens are to be connected to earth at one point only, usually the same point as the barriers

Barriers are the correct type and firmly fixed to earth bar

Record results by an effective and verifiable means

Inspection


Apparatus must be installed in accordance with drawings and schedules, fully labeled and undamaged

Segregation of IS and non-IS cable should be adequately maintained

Creepage and clearance distances must be acceptable where cable cores enter terminal blocks

IS earth connection must be separate from plant earth and other electrical earths, except at one point - the main electrical system earth point

Inspection


Where screens are used in IS cables then they should be earthed at one point only, usually at the same point as the interface devices are earthed.

In the field, screens should be isolated from earth and each other .

Cable armour or metal shields should be solidly earthed to the plant structure


Electrical inspection

Insulation testing (hazardous area)

Resistance of core to core and core to screen, typically

Pass > 10MFail < 1 M

Insulation testing (hazardous area)

Resistance of core to core and core to screen, typically

Pass > 10MFail < 1 M


Despite the fact that these are low current devices, they still generate 500V dc ( 500V rms ). If the cable has sufficient capacitance it has the potential to store enough energy to provide an incendive spark.

!Read the manual


Don't bridge the interface


V V Non-ISmeter

Simple test on barrier

Zener Diode Barrier


Work Live

Short circuit wiring on one loop at a time

Open-circuit wiring on one loop

Claim simple apparatus for:wire links as switchesresisters as loads

Perform measurements through barriers

You must not You mustUse the correct tools to avoid possible damage to enclosures etc.

Use certified test equipment

Use barriers for measurements with uncertified equipment

You may Interfere with the process

without authorisation

Compromise the safety of the loop by :

bridging the interface interconnecting separate

loops incorrect substitution of

interfaces invade IS circuits with

unrestricted power


Multiple Choice Questions on the topic of Intrinsic Safety

The standards indicate that a limitation is placed on the input voltage when supplying an instrument system via a zener barrier.The level stated in the standards is :

( a ) 55V ( b ) 110 V ( c ) 250V ( d ) 440V

When a screened cable is used to supply an intrinsically safe transducer , as part of an emergency shut - down system , itis recommended that the screen be terminated :

( a ) at one end only ( b ) at both ends

( c ) to the plant earth at both ends ( d ) isolated at both ends

A suitable instrument for measuring the resistive value of the copper protective conductor when used for intrinsic safety is :

(a ) an insulation tester ( b ) an ohmmeter

( c ) an audible continuity tester ( d ) a neon test screwdriver



The correct clearance distance between an IS circuit and earth , operating at a potential of 65V would be :

( a ) 1.5mm ( b ) 3mm ( c ) 4mm ( d ) 6mm

It is a recommendation that the unused cores of an instrument cable supplying intrinsically safe circuits , when terminatedin a control panel containing zener barriers be :

( a ) left to float in the enclosure until required ( b ) connected together and terminated to the plant earth

( c ) terminated in a connector and left to float until required ( d ) connected together and terminated to the intrinsically safe earth

An example of “ Simple Apparatus “ as defined in the standards would be :

(a ) a transducer producing an output of 120mA ( b ) a transducer producing an output of 2V

( c ) a light emitting diode ( d ) a capacitor capable of storing 30J



The minimum CSA of the protective conductor required for a circuit using a zener barrier to maintain the intrinsic safety wouldbe :

( a ) 1.5mm2 ( b ) 2.5mm2 ( c ) 4.00mm2 ( d ) 6.00mm2

Intrinsically safe and non - intrinsically safe circuits are permitted in the same enclosure provided that :

( a ) the cables have the same thickness of insulation ( b ) the cables are made by the same manufacturer

( c ) the cables are bound together in the same cable loom ( d ) the correct clearance is applied

Intrinsically safe components , circuits and systems are categorised as “ ia “ and “ i b “ . A device which is identified as thatwith the category “ ia “ indicates that it will remain safe with :

(a ) no faults ( b ) 1 fault

( c ) 2 faults ( d ) 3 faults


PORTABLE APPARATUS


Simple apparatus or any approved intrinsically safe apparatus with entity parameters

Vmax 30V

Imax 350mA

Ci + Ccable 0.02µF

Li + Lcable 0.07mH


R&H

Model 728+

I.S. Ground

C (F)Group L (mH)

0.11IIC 4

0.33IIB 12

Vt It

29.2V 297mA

Entity Concept parameters

Notes The barrier must not be connected to any device which uses or generates in excess of 250Volts RMS or DC unless it has been determined that the voltage has been adequately isolated from the barrier

The barrier must be connected to a suitable ground electrode. The resistance of the ground electrode must be less than 1 Ohm.


Hazardous area

24VApprovedbarrier

Barrier ParametersO/C voltage VbS/C current IbCapacitance CbInductance Lb

250 Approved apparatus

Max. Voltage VaMax. Current IaInternal Capacitance CaInternal Inductance La

Non Hazardous area

For this example Va , Ia Ca and La relate to the Apparatus . Likewise , Vb , Ib , Cb and Lb relate to the Barrier


Intrinsic Safety

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